CASTLE: NORTH AMERICAN RHYNCHOBDELLID^. 43 



ence, in the deeper parts of the body, of large, rounded cells each containing a 

 single yellow oil-drop, which is blackened when treated with osmic acid.^ 



Secondly, there are usually present (but this is the variable element in the 

 pigmentation) irregularly rounded, oval, or even somewhat branched cells, 

 which contain pigment granules either orange, dark-brown, or black in color. 

 These cells are found near the dorsal surface of the animal, and often produce 

 a conspicuous color pattern by their abundance in certain regions (Figure 38, 

 Plate 8). In their finer structure, cells of this variety are rather closely related 

 to the deep-seated pigment cells (reserve-food cells) found in G. stagnalis and 

 Gr. fusca; but in respect to position (close to the surface), and occasionally in 

 form (irregular or branched), they approach more nearly the superficial pig- 

 ment cells (" excretophores," Graf) of the species named. 



The pigmented areas which are often produced in G. heteroclita by the super- 

 ficial pigment cells just described are (Figure .38, Plate 8), first, a median dorsal, 

 longitudinal band, which, when best developed, extends, with occasional inter- 

 ruptions, from about the seventh somite back to the anus. In the anterior 

 ring of each somite it often broadens out into a trapezoidal form. Secondly, 

 in about the same regions of the body (seventh to twenty-seventh somites), 

 the anterior ring of each somite may be marked by a transverse, pigmented 

 line, most conspicuous a short distance from the margin of the body, from 

 which point it extends inward toward the trapezoidal, broad part of the median 

 vitta, but rarely joins it. 



Apdthy ('88) has recognized as a distinct variety (striata) animals which 

 have the transverse markings just described. It must be said, however, that 

 one can find in a lot of animals collected from the same locality all gradations 

 between forms "with no pigment at all (of the superficial sort) and those having 

 a median vitta and well-defined transverse striations. 



h. EiNGS, Somites, Eyes, Suckers, etc. 



The surface of the body is rather smooth, being only slightly rougher than 

 that of G. stagnalis. 



External rings, rather inconspicuous, particularly in the head region, where it 

 is often difficult to determine their number and limits accurately. 



Number of preanal rings, seventy, counting as a single ring each of the so- 

 mites I., II., XXVI., and xxvii.. Figure 19. This number may be increased, 

 if one counts subdivisions occasionally visible in some of the rings at the ends 

 of the body. 



Somites l. and ll., as just indicated, are commonly uniannulate (Figures 35, 

 36, Plate 8) ; but somite ii. is sometimes subdivided by a transverse furrow (as 

 shown in Figure 20, Plate 5). 



1 Fat cells are also found in the deep parts of the body of G. stagnalis, G. fusca, 

 and G. elongata, but the contained oil-drops are in those species perfectly clear 

 and transparent, so that they do not have the efEect of pigment cells, as do the fat 

 cells of G. heteroclita. 



